The invention relates to a vehicle headlamp comprising a light source.
The invention also relates to a vehicle provided with a headlamp.
Such headlamps are used on vehicles, such as cars, trucks, buses, bicycles and on vessels and aircraft.
Vehicle headlamps are known per se. A vehicle headlamp generally comprises an electric lamp with an incandescent body, for example, in a halogen-containing inert gas (a so-called halogen lamp) or an electric lamp with a pair of electrodes in an ionizable gas (a so-called discharge lamp). Such headlamps are customarily built up of two light sources which, in operation, generate either a so-called passing beam or a so-called main beam. Vehicle headlamps are known wherein the light source for the passing beam and the light source for the main beam are housed in a single lamp vessel (the so-called H4). Other known vehicle headlamps are those wherein two types of light sources are used, for example a halogen lamp in combination with a discharge lamp or in combination with a ring of light-emitting diodes. There are also headlamps which, in operation, generate colored light, which is generally brought about by a suitable coating provided on an outer surface of the lamp vessel. GB-A 2 200 199 discloses a vehicle headlamp the color of the light of which changes by switching from a first (colorless white) light source to a second light source with a color filter.
A drawback of the known vehicle headlamp is that the visibility of the part of the surroundings of the vehicle illuminated by the light source is insufficient.
It is an object of the invention to provide a vehicle headlamp of the type described in the opening paragraph, which headlamp provides an improved visibility of the surroundings of the vehicle.
To achieve this, the vehicle headlamp is characterized in accordance with the invention in that a luminous flux of at least one of the opto-electronic elements is at least 5 lm during operation, and in that the spectral characteristic of a light beam generated by the light source depends upon the position in the light beam.
Opto-electronic elements, also referred to as electro-optical elements, for example electroluminescent elements, such as light-emitting diodes (LED""s) with a luminous flux of 5 lm or more can suitably be used as a light source for vehicle headlamps. A relatively high luminous flux is necessary to ensure that also under ambient light conditions, for example sunlight or light originating from headlamps of other vehicles, sufficient light is generated so that a light beam generated by the light source can be observed sufficiently clearly from a distance.
To generate a so-called passing beam and/or a so-called main beam, a conventional vehicle headlamp requires a luminous flux ranging between 600 and 1000 lm to meet internationally standardized and specified light beam intensities. Current technology of opto-electronic elements, particularly that of light-emitting diodes, has yielded two different material systems which can suitably be used for different regions of the visible spectrum, i.e. Al In Ga N for blue-green light and Al In Ga P for yellow-red light. As a result, any desired spectral characteristic can be produced by combining suitable light-emitting diodes.
The use of a plurality of opto-electronic elements with a relatively high luminous flux enables a vehicle headlamp to be manufactured which provides an improved view of the surroundings of the vehicle. By making the spectral characteristic of the light beam generated by the light source dependent upon the position in the light beam, objects situated outside the center of the light beam are better observed. This observation of objects outside the center of the light beam is also referred to as off-axis viewing, as opposed to the so-called on-axis viewing, which refers to the visibility of objects situated on or in the vicinity of the axis of the light beam, in other words objects situated in the center or proximate to the center of the light beam (for example oncoming traffic). Examples of off-axis viewing are the observation of objects situated at or near the edge of the viewing field of the driver of the vehicle, for example the shoulder of the road, (unlit) objects, such as pedestrians or cyclists at the edge of the road on which the vehicle is traveling, and the observation of vehicles on a road which crosses the road on which the vehicle provided with the headlamp in accordance with the invention is traveling.
The known vehicle headlamp has two types of light beam, namely colorless white light or colored light, but each one of the light beams is generated by switching from a first light source to a second light source provided with a color filter. The individual light beams of the known vehicle headlamp do not demonstrate a substantial change in spectral characteristic as a function of the position in the light beam.
An embodiment of the vehicle headlamp is characterized in accordance with the invention in that the light source consists of a plurality of opto-electronic elements. Consequently, the vehicle headlamp is composed of one type of light source. By combining LED""s or so-called multi-chip packages having a luminous flux in the range from 10-250 lm, the luminous flux of a suitable combination of 25 such elements, or fewer, preferably a combination of 15 such elements, or fewer, such as a combination of four such elements, meets the international standard ranging between 600 and 1000 lm. The dimensions of a vehicle headlamp comprising such a relatively small number of opto-electronic elements are comparable to the dimensions of a conventional vehicle headlamp. An additional advantage of the use of LED""s is that the service life of these opto-electronic elements is very long as compared to that of the conventional vehicle headlamp.
In a preferred embodiment of the vehicle headlamp in accordance with the invention, the light beam comprises at least two beam segments having a substantially different spectral characteristic. In general, the intensity of the light beam emitted by a vehicle headlamp decreases as a function of the position in the light beam, i.e. the intensity is maximal in the center of the light beam and gradually decreases in the direction of positions situated at a larger distance from the center of the light beam. As a result, the intensity of the light for so-called on-axis viewing is relatively much higher than for the so-called off-axis viewing.
It is well-known that the sensitivity of the human eye changes with the wavelength of the light and that equal quantities of energy of different wavelengths generally cause different sensations of brightness. In other words, if we consider a spectrum, and the received energy (per unit of time) is equal for all wavelengths, the middle of the visible region will apparently be brighter than the red and the blue ends. If the intensity of the light is relatively high (luminous efficiencyxe2x89xa73.5 cd/m2), the maximum of the brightness lies at a wavelength of 555 nm (green/yellow), the so-called photopic eye-sensitivity curve. At this wavelength, particularly the cones are active in the human eye. If the intensity of the light is relatively low (luminous efficiencyxe2x89xa60.035 cd/m2), the maximum of the brightness lies at a wavelength of 507 nm (blue/green or cyan blue), the so-called scotopic eye-sensitivity curve. At this wavelength, particularly the rods are active in the human eye.
The inventors have recognized that it is advantageous to bring about substantially different spectral characteristics of the light beam for the on-axis and the off-axis parts of the light beam. In particular, in the off-axis parts of the light beam, the spectral characteristic of the light is adapted to a light intensity which is relatively low. By thus making the spectral characteristic of the light beam generated by the light source dependent upon the position in the light beam, the visibility of objects situated outside the center of the light beam is improved.
It is noted that, in practice, also photopic and scotopic light beams preferably have a broad spectrum, which light beams both have a more or less xe2x80x9cwhitexe2x80x9d appearance. The difference between a photopic and a scotopic light beam corresponds, as it were, to a difference between a light beam having a high and a low color temperature.
There is a possibility that the above-mentioned beam segments may comprise segments demonstrating a partial overlap. A possibly non-continuous transition between the beam segments may be perceived as unpleasant by the driver of the vehicle.
An embodiment of the vehicle headlamp in accordance with the invention is characterized in that the spectral characteristic of one of the beam segments is mesotopically tuned. The range between the photopic and the scotopic eye-sensitivity curve is referred to as mesotopic eye sensitivity curve (0.035xe2x89xa6luminous efficacyxe2x89xa63.5 cd/m2). By adapting the spectral characteristic of the light originating from the light beam of the vehicle headlamp in accordance with the invention, particularly in the off-axis parts of the light beam, to a spectral characteristic adapted to a light intensity which is relatively low, objects situated outside the center of the light beam are better observed. As a result thereof, the driver of the vehicle has a better view of the surroundings of the vehicle.
In a preferred embodiment of the vehicle headlamp in accordance with the invention, a first beam segment has a spectral characteristic which is rich in green-yellow light, and a second beam segment has a spectral characteristic which is rich in blue-green light. On the one hand, in the on-axis part of the light beam (the first beam segment), a spectral characteristic of the light is presented which is adapted to the photopic eye-sensitivity curve, i.e. rich in green-yellow light. On the other hand, in the off-axis parts of the light beam (the second beam segment), a spectral characteristic of the light is presented which is adapted to the scotopic eye-sensitivity curve, i.e. rich in blue-green light.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.